1. Technical Field
The present invention relates to a sheet-like gasket which is useful for sealing a flange connection portion of a pipe joint, a passage connection portion of a fluid apparatus, or the like, and more particularly to a sheet-like gasket formed of a material having a layered internal structure such as expanded graphite or porous polytetrafluoroethylene (PTFE).
2. Prior Art
Sheet-like gaskets of such types which are conventionally known include those having the following structures:
(A) A gasket which, as shown in FIG. 24, is produced by forming a material having a layered internal structure such as expanded graphite superior in heat resistance, airtightness and stress relaxation property, or porous PTFE superior in contamination preventing property for fluid, into a sheet-like shape by pressurization, and then stamping the sheet-like material into an annular shape, so as to form a sheet-like base member 10 having a predetermined shape. PA0 (B) A gasket such as a print gasket in which, as shown in FIG. 25, local protrusions 11 are formed on both front and back faces 10a and 10a, i.e., on tightening faces of a sheet-like base member 10 having a predetermined shape made of expanded graphite, porous PTFE, or the like, so that a high face pressure is locally applied to the contact surface. PA0 (C) A gasket in which, as shown in FIG. 26, inside and outside portions of a sheet-like base member 10 having a predetermined shape made of expanded graphite, porous PTFE, or the like are compressed from both front and back faces for forming recesses, so that a high-density layer 13 is formed between the recess portions 12 and 12 on the front and back faces. PA0 (D) A gasket in which, as shown in FIG. 27, an inner circumferential face portion of a sheet-like base member 10 having a predetermined shape made of expanded graphite, porous PTFE, or the like is compressed by using a mold, etc., so as to form a high-density layer 13. In FIG. 27, the high-density layer 13 is formed only on the inner circumferential face portion of the sheet-like base member 10. Alternatively, the high-density layer 13 may be formed only on an outer circumferential face portion, or on both inner and outer circumferential face portions. PA0 (E) A gasket in which, as shown in FIG. 28, on an outer or inner end portion of a sheet-like base member 10 having a predetermined shape made of expanded graphite, porous PTFE, or the like, a local sheet base member 14 made of the same material as that of the sheet-like base member 10 is overlapped, and then pressurized so that the thickness of the whole structure is uniform. In FIG. 28, the local sheet base member 14 is overlapped only on the outer end portion of the sheet-like base member 10. Alternatively, the local sheet base member 14 may be overlapped only on the inner end portion. PA0 (F) A gasket produced by, as shown in FIG. 29, coating both front and back faces of a sheet-like base member 10 having a predetermined shape made of expanded graphite, porous PTFE, or the like with coating members 15 and 15 made of PTFE or the like, or by, as shown in FIG. 30, coating the entire face of the sheet-like base member 10 with a coating member 15 made of PTFE or the like. PA0 (G) A gasket by, as shown in FIG. 31, covering and enclosing an inner end portion and an inner circumferential face portion of a sheet-like base member 10 having a predetermined shape made of expanded graphite, porous PTFE, or the like with a covering member 16 made of a metal, PTFE (which is not porous), or the like.
In general, in a sheet-like gasket in which a sheet-like base member 10 is formed of a material having a layered internal structure such as expanded graphite or porous PTFE, as is apparent from an enlarged cross-sectional structure shown in FIG. 32, both front and back face portions 10a and 10a of the sheet-like base member 10 are in a high-density and high-alignment (high alignment: a state in which alignment in one direction is attained, so that, for example for expanded graphite, flaky expanded graphite particles are tidily arranged in a stacked manner) state. Accordingly, as indicated by b in the figure, leakage from the inside to the front and back face portions 10a and 10a hardly occurs, but, as indicated by a in the figure, penetration leakage via the internal layer to the outside tends to easily occur.
With the aforementioned view points, the above-listed conventional sheet-like gaskets will be individually studied. In the structures of (A) and (B), leakage from the contact surface, i.e., contact-surface leakage can be suppressed or prevented by tightening at a high face pressure, but penetration leakage such as that indicated by a in FIG. 32 cannot be prevented from occurring. In the structures of (C) and (D), penetration leakage is reduced due to the presence of the high-density layer 13, but contact-surface leakage easily occurs. Furthermore, in the structure of (E), both contact-surface leakage and penetration leakage can be reduced because the passage for penetration leakage is narrow. The narrow passage, however, does not mean that the passage for penetration leakage is cut off. Thus, a small amount of penetration leakage cannot be prevented from occurring, so that high sealing properties cannot be attained in any way.
In the structure of (F), particularly in the structure wherein the entire face is coated, the density of the coating member portions on the front and back faces is increased as the degree of the tightening is advanced, so that contact-surface leakage can be surely prevented. However, the density of the coating member portions on the inner and outer circumferential faces is not increased, so that penetration leakage cannot be prevented from occurring. In the structure of (G), it is desired to use a metal or hard non-porous PTFE as the covering member 16 in order to prevent penetration leakage. However, in some cases, for example, in the case where the tightening face pressure cannot be made higher, in the case where a joint flange is soft, or in the case where the gasket is used at a high temperature, a hard material cannot be used depending on the circumstances in which leakage is to be prevented. In such cases, it is necessary to use a covering member 16 made of a soft material having heat resistance and corrosion resistance, so that penetration leakage inevitably occurs.